A sheet comprising a fiber assembly, typically a nonwoven fabric, which includes extra-fine fibers having a fiber diameter of nanometer size to micrometer size, has been used in a wide range of applications such as those of separators or electrolyte membranes of lithium secondary batteries, separators of fuel batteries, filters and medical fields.
As a method for preparing a fiber assembly including extra-fine fibers having a fiber diameter of nanometer size, an electro-spinning method is known (see, for example, Patent Document 1). In this method, when a polymer solution or a polymer melt is extruded from a spinning nozzle, a high voltage is applied between the spinning nozzle and a counter electrode to accumulate charges in a dielectric material in the nozzle, thereby producing extra-fine fibers by means of an electrostatic repulsive force. In Patent Document 1, by using a highly volatile solvent as a solvent or by elevating a temperature of a polymer solution, the viscosity of the polymer solution is reduced without significantly reducing the concentration of the polymer so as to suppress thickening of fibers.
Patent Document 2 discusses to obtain a sheet including a nonwoven fiber assembly in a fabric shape by electro-spinning a spinning dope which is prepared from a fiber-formable organic polymer in addition to a proton conductive polymer (see, for example, Patent Document 2).